Much of the body of a Pederson’s transparent shrimp looks like watery nothing, but it’s a superhero sort of nothing. The shrimp is transparent enough to read through, but it’s not some frail, filmy thing. It’s packed with invisible muscle.

Searching for Ancylomenes pedersoni shrimp has a touch of the summer-camp prank about it, being a hunt for something that’s mostly invisible. On a research trip to reefs in Belize, “everyone else would give up,” says Laura Bagge of Duke University. But she’d...